Projection lamp with elliptical mirror and light source mounting structure



J. DE RIDDER ETAL 3,488,543 PROJECTION LAMP WITH ELLIPTICAL MIRROR AND LIGHT NG STRUCTURE Jan. 6, 1970 SOURCE MOUNTI Filed Jan. 25, 1968 Fla-4' FIG.2

HOY M B Amvr United States Patent Office 3,488,543 Patented Jan. 6, 1970 Int. Cl. H01j 65/08, 17/32 US. Cl. 313-113 3 Claims ABSTRACT OF THE DISCLOSURE A halogen projection lamp having an elliptical condensing mirror on the bulbs inner surface, and a helically wound filament having a length-to-diameter ratio less than 1.5 disposed at a focus point of the mirror, with the axes of the filament and mirror coincident.

The invention relates to a combination 'of a condenser mirror and a light source for the illumination of the film gate of a film projector, the reflective surface of this condenser mirror having the form of part of an ellipsoid of revolution and the light source having the form of a helically wound filament which contains one of the two foci of this ellipsoid and the axis of which substantially coincides with the axis of revolution of the mirror.

Such a combination is known in the form of a projection lamp, a mirror'coated bulb part of which serves as condenser mirror. The bulb of this lamp is closed by a lamp cap in which the current-supply wires of the filament are incorporated in a usual manner. In this known lamp, the cap is located at the top of the ellipsoidal mirror, that is to say at the point at which the axis of revolution of the mirror intersects the same.

Such a lamp has the advantage that an accurate orientation of the light source with respect to the mirror is obtained already during the manufacture of the lamp and is therefore independent of the manufacturing tolerances in the construction of a projector in which the lamp is to be used. Other known combinations, in which the light source and the mirror are separately secured in the projector, do not exhibit this advantage.

The orientation of the filament along the axis of revolution of the mirror results in that a large part of the light flux emitted by the light source can be collected by the mirror. With differently orientated filaments, the part of the light flux collected by the mirror will generally be smaller. This is also due to the fact that with this orientation, the comparatively dark terminal apertures of the filaments face the lamp cap and the mirror opening, respectively.

The known incandescent lamp described has the disadvantage, however, that it is difiicult or impossible to use a halogen incandescent lamp. Under suitable operating conditions of an incandescent lamp, the addition of a halogen to the bulb filling of this lamp results in a chemical conversion cycle regenerating the filament so that the life and the light output of the lamp are considerably improved due to the size of the bulb required for normal projection applications where the bulb serves as a mirror.

This disadvantage could be avoided by combining without further expedients a separate halogen incandescent lamp with a condenser mirror, but careful attention must then constantly be paid to the accurate relative orientation of the mirror and the lamp in the projector. Moreover, the small bulb of the halogen incandescent lamp exhibits comparatively large unevennesses-such as a pinch in which the current-supply wires are secured and a sealing tip of an exhaust tube used in the manufacturewhich are liable to disturb the indispensable uniformity of the light distribution over the film gate illuminated by the said combination.

However, experiments have shown that nevertheless a combination of the said kind can be constructed which does not or substantially does not exhibit the aforementioned disadvantages. According to the present invention, this combination is characterized in that the rotationsymmetrically wound filament is disposed inside the bulb of a halogen incandescent lamp which is firmly secured to the condenser mirror, the ratio between the length and maximum diameter of the 'helically wound part of the filament being smaller than 1.5. Thus, the advantage is maintained that the light source and the mirror are relatively accurately orientated already during the manufacture so that this orientation is not influenced when the combination is mounted in a projector. When the filament is wound rotation-symmetrically and when the dimensions of the filament are chosen in accordance with the invention, it is moreover found that a combination can be manufactured which includes a mirror having a size suitable for film-projection applications (for example, a mirror having a maximum diameter of the order of 5 cm.), the high light output of a halogen incandescent lamp being utilized and a high-quality light distribution (over a film gate or projection cloth illuminated by this combination) being obtained. The unfavourable influence of the unevennesses of the lamp bulb can be considerably suppressed by the steps according to the invention when the lamp and the mirror are relativel accurately orientated. With the ratio between the dimensions of the filament chosen in accordance with the invention, the combination is found to have a high light output per unit of supplied power. When using the steps according to the invention, with a suitable proportioning of the apertures of objectives of film-projection apparatus, the dimensions of the filament of the combination according to the invention used in such a projector can be chosen so that the aperture of the objective of this projector is substantially filled by the light beam to be emitted by this combination through the film gate of the projector. Consequently, the objective is fully utilized.

In order to further increase the light output of the combination according to the invention, the filament may be wound from covered wire.

The accurate relative orientation of the light source and the mirror during the manufacture of a combination according to the invention may be obtained in an ei'ficacious manner, for example, by means of an apparatus by which the light distribution brought about on a screen by the combination through a film gate and an objective is measured. The image surface on the screen and the film gate may have the usual rectangular shape. It is found in general that with an optimum orientation of the light source and the mirror of a given combination, the light distribution over such a rectangular image surface depends also on the angular position about the optical axis of the measuring device occupied by this combination during the measurement. This is due to the fact that a certain asymmetry of the construction of the halogen lamp (such as the form and the relative positions of the pinch, the current-supply wires, the bulb, the sealing tip) and manufacturing tolerances of mirror and lamp cause the combination once orientated to the optimum to be not entirely rotation-symmetrical either. Therefore, in order to further utilize the optical properties of a combination according to the invention, it is desirable that, when mounting the combination in a projector, the angular position which it occupies during the orientation should be reproduced with the greatest possible accuracy.

As is common practice for securing concave mirrors J in a projector, in a favourable embodiment of the combination according to the invention, the condenser mirror 7 is provided with an annular fitting rim which extends transversely of its axis of revolution and is used for centering the combination with respect to the optical axis passing through the film gate of the projector, so that the angular position of the combination about this optical axis which is the optimum for the light distribution can be fixed in a simple manner. According to the invention, this embodiment is characterized in that the mirror is provided at or in the proximity of its fitting rim with a mark, preferably in the form of an abutment cam or groove. As a matter of course, during mounting of the combination in a suitable projector, such a cam or groove should co-operate with an associated groove or cam of said projector.

The invention will now be described with reference to the drawing, which shows only one embodiment of a combination according to the invention.

FIGURE 1 is an elevation of this combination, the mirror being shown in a longitudinal sectional view.

FIGURE 2 is an elevation similar to that of FIG- URE 1, but viewed in a direction which is shifted by 90 with respect to the direction of view of FIGURE 1. In FIGURE 2, the mirror is shown in a longitudinal sectional view taken on the line XX of FIGURE 1.

FIGURE 3 is a front elevation and FIGURE 4 is a rear elevation of the combination.

The embodiment shown comprises a concave condenser mirror body 1 (in this case made of glass) which has on the inner side an ellipsoidal surface which is provided with a thin reflective layer 2. This reflective layer 2 preferably acts as a so-called cold-light mirror. The axis of the ellipsoid of revolution (at the same time the optical axis of the combination) is denoted by XX and one of the two foci by F. When used in conventional film projectors, the maximum diameter D of the mirror will be of the order of 5 cm. The filament 9 of the halogen lamp 3 is helically wound, preferably from covered wire, and in this embodiment has a cylindrical shape. It is arranged so that it contains the focus F and that its axis coincides with the axis XX of the ellipsoid. The filament 9 is suspended in the bulb 4 of the halogen lamp 3 by means of the stay wires 8 which are secured to the foils 6 received by the pinch 5 of the lamp. Furthermore, the current-supply pins 7 are connected to the foils 6. The bulb 4 also has the sealing tip 10. After being accurately orientated with respect to the mirror, the lamp is permanently secured in the neck 14 of the mirror body 1 by means of a suitable cement material 11.

The helically wound part of the filament 9 has a length l and a maximum diameter d. According to the invention, the ratio between I and d is smaller than 1.5. Also due to this fact, the advantages mentioned in the preamble with respect to the light output and the uniform light distribution over an image surface illuminated by the combination can be obtained in the combination according to the invention.

The mirror body 1 is further provided with an annular fitting rim 12 which, when the combination is secured in a suitable projector, ensures that the combination is satisfactorily centered with respect to the optical axis of the projector. The fitting rim 12 is provided with an abutment cam 13 which indicates the angular position about the optical axis XX in which during manufacture the lamp and the mirror of the combination are relatively orientated so that the combination actually brings about a light distribution of the greatest possible uniformity over an illuminated image field. Of course, when the combination is mounted in a projector, the abutment cam 13 should co-operate with an associated groove or opening in this projector so that the said optimum angular position of the combination is reproduced with the optimum accuracy.

What is claimed is:

1. In a halogen incandescent lamp for use with a film projector to illuminate the film gate thereof, the improvement in combination with the bulb thereof comprising:

(a) the inner surface of the bulb formed as an ellipsoid of revolution with a maximum diameter of about 5 cm.;

(b) a reflective surface formed on said inner surface as an elliptical condensing mirror, and

(c) a helically-wound filament light source secured to the mirror, the filament being at one of the foci thereof, the axis of the filament and mirror being coincident, and the length-to-diameter ratio of the filament being, smaller than 1.5.

2. A lamp as defined in claim 1 wherein the bulb has a pinch where it was sealed, the pinch being disposed along said axis of the filament.

3. A lamp as defined in claim 1, further comprising on the condenser mirror an annular fitting rim extending transversely of the mirrors axis of revolution and used for centering the mirror with respect to the optical axis passing through the film gate of the projector.

References Cited UNITED STATES PATENTS 2,233,486 3/1941 Portnow 24041 3,365,604 1/1968 Dayton et al 313 X 3,381,575 5/1968 Levin 313-113 X 3,390,262 6/1968 'Elmer 2401l.4 X

OTHER REFERENCES I. Koch: The Ellipsoidal Reflector, Electronic Engineer, July 1967, p.56.

JAMES W. LAWRENCE, Primary Examiner DAVID OREILLY, Assistant Examiner US. Cl. X.R. 313220, 222 

1.5 DISPOSED AT A FOCUS POINT OF THE MIRROR, WITH THE AXES OF THE FILAMENT AND MIRROR COINCIDENT. 